This application is a continuation-in-part of International application PCT/GB98/03849, filed Dec. 21, 1998, and with a claim of priority from original UK application No. 9727367.6, filed Dec. 24, 1997.
This invention relates to apparatus useful for controlling and/or monitoring the inflation pressure within a laryngeal mask airway (LMA) device and to a method and means for monitoring a patient's welfare by way of fluctuations in that inflation pressure.
LMA devices are now well known and are in widespread use in patient care, both during and after surgical procedures and during procedures that involve maintaining a clear airway. Such devices and their construction and use are described in various patent publications, for example, British Patent No. 2,205,499 and U.S. Pat. Nos. 4,509,514; 5,303,697; 5,241,956; and 5,282,464.
That such LMA devices are well accepted tools in patient care is borne out by simple statistics. For example, it is estimated that such devices are used in approximately 50 percent of all operative procedures requiring the use of general anaesthetic in the UK, and their use is becoming increasingly accepted elsewhere in Europe and in the USA.
Basically, an LMA device comprises an airway tube that is sized and curved for general conformance with a patient's airway; the airway tube extends from a proximal end that is external to the patient, to a distal end that carries mask structure in the form of a bowl or backing plate which faces and covers the patient's laryngeal inlet and which is continuously surrounded by a flexible ring or cuff that is selectively inflatable for resiliently sealed conformance to body structure around the laryngeal inlet. In use, the LMA is first fully deflated to aid its insertion and is then passed through the patient's mouth and throat into its correct position, with the mask over the laryngeal opening. The inflatable cuff surrounding the mask structure is then inflated to form a seal between the mask and the laryngeal opening. The air pressure with which the cuff is inflated forces the rear of the bowl of the mask against firm tissues at the back of the throat to maintain the device in place and to retain the seal. In this way, the device forms an open airway through which the patient's lungs can be ventilated.
Such LMA devices have proved to be both sturdy in construction and relatively straightforward to use, even by paramedics after the necessary training; however, if a mask is wrongly inserted, a reliable airway is not in fact formed and the patient's lungs are not then properly ventilated. In addition, the soft compliant silicone material of the cuff has been known to absorb nitrous oxide from anaesthetic gas mixture thereby increasing the pressure within the cuff, and the cuff itself may occasionally develop leakage causing its internal pressure to decrease. Also, it has been found that too high an inflation pressure will cause the cuff to restrict the blood supply to the mucosa overlying the muscles around the laryngeal inlet, and prolonged use of an LMA device in such circumstances can lead to tissue necrosis.
It has recently been proposed to monitor a patient's level of consciousness by means of a device capable of detecting and analyzing cerebral activity, in an attempt to avoid awareness during surgery; however, the equipment required for such monitoring is complex in operation and costly to manufacture. It has also been proposed to use automatic equipment to monitor intra-cuff pressure in a high-volume, low-pressure cuff associated with an endotracheal tube (EDT). And it has been reported that such an EDT device is able to measure and regulate intra-cuff pressure, to a tolerance of ±3-mm mercury.
We have observed that muscle tension (tone) in the lower throat (hypopharynx) bears a relationship to the patient's apparent response to painful stimuli; thus, it is possible to gain information on the patient's anaesthetic depth by monitoring muscle tone in the hypopharynx. And we have found that very small changes in that muscle tone are reflected through the LMA cuff and are further reflected through the shared air volume and that such pressure feedback readings can be utilized to both automatically adjust the inflation pressure of the LMA cuff and also to detect changes in the muscle tone in the hypopharynx. Of course, the detection of such changes is most preferably made without causing patient duress, and to such end it has been observed that it is most beneficial to inflate the cuff to pressures not much higher than 60-cm H2O; for example, 50-cm H2O, and generally in the range 25-cm H2O to 100-cm H2O. The acceptability of utilizing pressures above 60-cm H2O has been observed to cause patient throat irritation, especially in procedures of longer duration, and may risk damage to such tissue.